1. Field of the Invention
This invention is related particularly to batch furnace methods using arc column forming plasma generators for heating gases used in annealing of softening coiled rod, wire, strip and similar metals.
2. Description of the Prior Art
It is a well-known practice in the production of ferrous and non-ferrous sheet, rod and wire to subject the product to heat treatment for varying lengths of time and under varying atmospheric conditions. Such heat treatment may be adapted to effect certain physical changes as in normalizing and annealing, or by heating the product in the presence of a controlled reactant atmosphere to effect certain solid-state chemical changes as in carburizing, decarburizing, nitriding, denitriding, oxidizing, reducing, etc.
The practice of annealing low carbon sheet or strip steel, for example, has included continuous as well as batch-type methods of annealing. In continuous annealing, a steel strip is annealed while passing as a single strand through a furnace. The furnace atmosphere may or may not be "controlled". The degree of softening obtained is governed by the maximum temperature of the strip. The maximum temperature of the strip in turn depends on the energy radiated from heat sources, the thickness of the strip, and the rate of transit through the furnace. A continuous normalizing furnace without atmosphere control may use burners whose products of combustion play directly on steel sheet but produce a scale that can be tolerated. It has not been known, however, so far as applicants are informed, to use such products of combustion and at the same time to obtain a scale-free steel. Because transit time through the furnace limits the temperature to which the strip can be heated and hence limits the strip thickness, and because the relatively rapid cooling of a continuously annealed strip imparts higher hardness and a quench aging tendency than does batch-type annealing, the continuous annealing process is limited to light gauge product of restricted use. The major portion of present ferrous sheet and strip heat treating is, therefore, carried out in batch-type furnaces.
The conventional and widely used batch-type furnace comprises one or more stationary "diffuser bases" which house a recirculating fan and on which the charge to be annealed or otherwise heat treated is supported. A cylindrical removable steel inner cover encloses the charge, and an outer refreactory lined cover is lowered over the assembly. The outer refractory lined cover serves as a thermal barrier during heating and permits a controlled cooling cycle. The relatively thin inner cover dissipates and transfers heat rapidly, confines the controlled atmosphere during heating and preserves the controlled atmosphere during cooling until the temperature of the charge is sufficiently low to prevent scaling when exposed to ambient air. The strip steel is ususally tightly wound around a vertical mandrel and the resulting "hard wound" coils may be stacked on top of each other. Rod is wound around a similar mandrel and several such coils may be placed adjacent one another on the diffuser base.
In an alternate batch-type heat treating practice, sheet steel is loosely wound around a vertically disposed mandrel, each lap being separated from adjacent lap by a wire or nylon cord separator. Because the entire surface of such an "opened" coil is exposed to a gas of known and controllable composition, annealing practices, for example, have also included changing the chemical composition of the coil by solid state reactions during the "annealing" process by admitting certain reactant gases, e.g., moistened hydrogen, to the treating chamber.
In either case of hard coil or open coil batch-type annealing, the lift-off cover is in place about the inner cover during the heat-up, during the soak period, and during a portion of the cooldown cycle, if uncovering of the inner cover at the end of the soak period would result in too rapid a cooling rate or dangerous exposure of the surroundings to excessive heat. At the end of the cooling cycle, the inner cover is kept over a steel charge until the inside temperature has dropped sufficiently to ensure an oxide-free steel surface on exposure to ambient air. It is to be noted that the same high convection protective atmosphere batch-type annealing equipment is in widespread use in the rod and wire industry. Also, note that some batch annealing furnaces have stationary outer covers and raise and lower the hearth floor to place the inner cover and and charge within the outer cover.
In batch furnaces according to the prior art there is supplied a controlled atmosphere gas to the volume enclosed by the inner cover which in turn is heated by gas-fired radiant tubes or direct-fired or semi-direct-fired burners which line the interior lift-off cover and heat the inner cover. The thermal energy required to heat the charge from ambient to a selected high temperature must pass through the annular space between the outer cover and the inner cover, through the wall of the inner cover where it is transferred to the controlled atmosphere gas, and then to the charge. Convection, radiation and conduction heat transfers are involved. Much energy is lost in the above process. It is widely known that in conventional batch furnaces thermal efficiencies, i.e., the fuel energy that reaches the charge, are limited to about 50% even when using radiant gas-fired tubes which have been reported to be the most efficient source of heat energy in furnaces of this kind. Due to the relatively inefficient method of heating the charge, a substantial thermal head must be maintained in conventional annealing furnaces. By this is meant that the temperature at the radiant tubes must be maintained substantially higher than the temperature in the inner cover. Annealing practice in batch furnace operation calls for a furnace control period during which the charge is brought up to a work temperature. As an example, a radiant tube gas temperature in excess of 1800.degree.F is normally required to heat the inner cover atmosphere to a work temperature of 1275.degree.F. The time varies between furnaces and different charges, but generally requires 10-20 hours. Thus, a substantial amount of furnace time is involved in heating the inner cover atmosphere in conventional annealing batch furnaces, prior to the soak in the temperature cycle.
U.S. Pat. No. 3,109,877 is directed to an apparatus for heat treating loosely wound metal coils. A gas-fired tube or electrical resistance heat source heats a volume of controlled atmosphere gas which is fan driven into an open coil treating chamber. While open coil heat treatment of ferrous sheet is widely used in the steel-making industry in conjunction with lift-off batch furnaces of the above described class, such an apparatus for heating the controlled atmosphere gas, as disclosed in the above U.S. patent has not been commercially successful for a number of practical reasons but primarily due to the substantially low thermal efficiencies which are obtained from heating the controlled atmosphere gas with conventional gas burners and electrical resistance coils.
Related to the above discussion, it should also be recognized that the steel industry has used batch-type furnaces since the early 1930's but there has been no substantial change in the methods and apparatus used to heat and control the annealing atmospheres in the inner covers of the batch furnaces. In other areas of steelmaking concerned with reduction and melting processes, it has long been known to use an electric arc as a heat source. See, for example, U.S. Pat. No. 1,479,662. A more recent innovation in the steel industry in the United States, Germany, Russia and Japan has been the introduction of furnace wall or internally mounted plasma arc generators for use in melting and refining wherein the plasma electric arc has been employed as a source of heat in melting and in liquid state refining processes. In this connection, reference should be made to previously cited copending applications, to German Pat. No. 1,206,531 having an "Armeldetag" date of May 28, 1963, and U.S. Pat. Nos. 3,422,206; 3,496,280; and 3,524,006. The employment of a plasma generator within a vessel for vessel space preheating has also been recognized in the previously referred to copending application Ser. No. 283,514 in which the space heated has no relation to a controlled atmosphere. None of these references or any other known references dealing with employment of electric arcs and more specifically with plasma arcs have suggested any application of an externally mounted plasma generator in connection with heating and controlling an atmosphere in a batch-type annealing furnace for solid-state heat treatment and chemical modification. More specifically, none of such references has suggested the possibility of converting conventional batch furnaces from fossil-fired fuel operations in which the inner cover atmosphere is indirectly heated over a long period of time to a system in which a plasma gas is heated externally of the furnace and the same gas is used both to sustain the plasma arc and to provide an atmosphere treating gas which can be introduced and brought up to a temperature near the working temperature in the inner cover within a matter of minutes as compared to the hours of time heretofore required.
The invention in one aspect directs itself to a method of converting a conventional fossil fuel fired batch furnace. Therefore, it is appropriate to recognize that others in the prior art have converted fossil fuel fired heating apparatus to electrically heated apparatus and in this regard reference is made to U.S. Pat. No. 3,691,344. However, neither this reference nor any other similar reference known to applicants makes any reference to the specific subject matter of this invention; namely, that of converting a fossil fuel fired batch-type annealing furnace for treating metals in a solid state to a furnace utilizing direct heated plasma gas as the atmosphere gas. The overall subject of batch furnaces has been widely reported as well as the critical features concerned with furnace atmospheres. Reference is made to the publication "Recent Developments in Annealing", Special Report 79, published in England, 1963, by the Iron and Steel Institute. This publication discusses the practices of the industry in batch annealing as of this date and such practices have generally not changed since such date. The many critical features concerned with furnace atmospheres are described in the publication "Furnace Atmospheres and Carbon Control", published by the American Society for Metals in 1964. A sales Bulletin LW1255, published by the Lee Wilson Engineering Company, Inc., of Cleveland, Ohio, shows in some detail typical batch furnaces being employed for rod and wire annealing. controlled atmosphere furnaces of various kinds are also described in a sales leaflet identified as Form No. SC-97 published by the Surface Combustion Corporation, Toledo, Ohio, and entitled "The ABC's of Prepared Atmospheres". Another useful reference to illustrate typical conventional batch furnace operation when directed to open coil annealing is to be found in the article entitled "Use of Open Coil Process to Change Composition and Improve Sheet Steels" to be found in the publication "Iron and Steel Engineer", May 1961. U.S. patent references dealing with the subject of batch or box annealing include U.S. Pat. Nos. 2,602,034; 2,603,577; and 3,127,289.
From the foregoing description of the prior art, those skilled in the art will recognize that batch furnace constructions and methods of operation have basically remained static since their introduction in the 1930's and even though electric arc and plasma generated arcs have made their appearance in other phases of steelmaking there has been no recongition or suggestion, prior to the present invention, that plasma arc gases can be used both as an atmosphere gas for annealing as well as a gas to sustain the plasma arc and that an external mounted plasma generator can be used as a basis for converting conventional fossil fuel fired batch furnaces to an entirely different mode of operation. The known and well recognized disadvantages of batch furnaces include the difficulty of maintaining uniform temperature, hot spot overheating, refractory maintenance, inner cover maintenance, low fuel efficiency, ignition explosions, limited inner cover life, and disposing of combustion products. Yet, since the early 1930's there has been no substantial way of avoiding or minimizing these problems and disadvantages and such becomes the object of this invention.